Carbon steel spring elements

ABSTRACT

A CARBON STEEL COMPOSITION FOR SPRING ELEMENTS IS DISCLOSED WHEREIN THE STEEL INCLUDES CARBON, MANGANESE, FILICON AND BORON. IN ACCORDANCE WITH ITS CLASSIFICATION AS A &#34;CARBON STEEL,&#34; IT DOES NOT INCLUDE NORMAL ALLOYING ELEMENTS, PARTICULARLY CHROMIUM. THE PRESENT STEEL IS PARTICULARLY CHARACTERIZED BY EXCELLENT DURABILITY AND FATIGUE LIFE AS WELL AS BY LOW COST. THE CARBON STEEL CONSISTS ESSENTIALLY OF ABOUT .50-.80 PERCENT CARBON, ABOUT 0.50-1.65 PERCENT MANGANESE, ABOUT 0.02-0.60 PERCENT SILICON, ABOUT 0.050 MAXIMUM PERCENT SULFUR, ABOUT 0.040 MAXIMUM PERCENT PHOSPHORUS, ABOUT 0.0005-0.007 PERCNET BORON, BALANCE MAINLY IRON.

United States Patent 3,689,329 CARBON STEEL SPRING ELEMENTS Roy F. Kern,Peoria, Ill., assignor to Caterpillar Tractor Co., Peoria, Ill. NoDrawing. Filed Aug. 20, 1969, Ser. No. 852,157 Int. Cl. CZId 9/02 U.S.Cl. 148-36 3 Claims ABSTRACT OF THE DISCLOSURE The present inventionrelates to a novel'carbon steel composition which has been foundsuitable for use in spring elements.

Spring elements such as coil springs, leaf springs, torsion bars, etc.have conventionally been made from alloy steels within the prior art.This preference appears to have arisen out of the general belief thatcarbon steel springs could not be quenched and tempered to high strengthlevels in sizes much greater than 0.250 inch in diameter or thickness.Accordingly, carbon steel has not been considered for use in springelements since it was believed that the weight and size of the springelements would have to be inordinately great to achieve adequatestrength standards.

As a basis for the present invention, it has been found that carbonsteels may be employed to produce very satisfactory spring elementsthrough the addition of boron and the control of various other elementswithin the steel composition.

It is important to note that the present invention is concerned onlywith carbon steels and not with alloy steels of the type which has beenused for spring elements in the prior art. It therefore appears usefulto include herein the standard definitions for alloy steels and carbonsteels, as defined for example by the American Iron and Steel Institute.

An alloy steel is defined by the American Iron and Steel Institute asfollows:

When the maximum range given for the content of alloying elementsexceeds one or more of the following limits: manganese 1.65 percent,silicon 0.60 percent, copper 0.60 percent or in which a definite rangeor a definite minimum quantity of the following elements is specified orrequired Within the limits of the recognized field of constructionalsteels: aluminum, chromium up to 3.99 percent, cobalt, columbium,molybdenum, nickel, titanium, tungsten, vanadium, zirconium-or any otheralloying element added to obtain a desired alloying effec A carbon steelis defined by the American Iron and Steel Institute as follows:

No minimum limit for aluminum, chromium, cobalt, columbium, molybdenum,nickel, titanium, tungsten, vanadium, zirconium-or any element to obtaina desired alloying effect. Specific minimum copper shall not exceed 0.40percent. Maximum content for any of the following elements shall notexceed the percentages noted: manganese 1.65, silicon 0.60, copper 0.60.

To provide adequate strength in most spring elements,

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which are subject to very high fatigue loading, it appears that at leastapproximately 0.50 percent carbon is necessary to provide optimum life.Prior to the present invention, it was generally held that boron couldnot contribute significantly to hardenability as would chromium, forexample, because the boron effect was thought to decrease sharply withincreasing percentages of carbon.

Contrary to this general belief, it has been found as a basis for thepresent invention that boron is very effective in increasinghardenability of medium to high carbon spring steels of the type setforth herein.

A very basic advantage for the present carbon steel in the manufactureof spring elements lies in the reduction of costs which it makespossible. This cost reduction is made possible through the eliminationof various alloying agents of which chromium is a particularly importantexample. In addition, manufacture of spring elements from the presentcarbon steel compositions is made independent of elements such as theabove alloying agents which tend to be relatively scarce. On the otherhand, the elements present within the steel composition of the presentinvention tend to be available in plentiful supply.

Still further, spring elements manufactured from the present carbonsteel exhibit at least equivalent static strength and fatigue strengthas compared to spring elements manufactured from conventional alloysteels. It further appears that spring elements manufactured accordingto the present invention exhibit certain advantages over alloy steels.One outstanding advantage for carbon steels lies in its shell hardeningcharacteristics to provide endurance limit fatigue strength. A secondadvantage is the fracture toughness of carbon steels according to thepresent invention. Because of these advantages, spring elements formedfrom the present carbon steel composition may have increased durabilityand fatigue life as compared to spring elements formed from conventionalalloy steels.

Additional important considerations for the manufacture of such springelements lies in the ease with which carbon steels may be cold formed,forged, machined, welded, and heat treated under proper parameters.These characteristics further contribute to the facility with which thespring elements may be manufactured and to their finished physicalcharacteristics.

It is accordingly an object of the present invention to provide carbonsteel compositions suitable for manufac turing spring elements.

It is a further object to provide such compositions through the additionof boron to medium to high carbon steels.

It is a further object of the present invention to provide carbon steelspring elements with high durability and fatigue life.

It is still another object to provide spring members formed from carbonsteel comprised of carbon, manganese, silicon and boron.

Carbon steel compositions of the present invention are comprised of ironin large part. Other elements which are particularly considered withinthe composition include carbon, manganese, silicon, and boron as well asmaximum percentages for sulfur and phosphorus.

The useful approximate range of the above noted elements in the steelcomposition are as follows:

Element: Percent Carbon 0.5-0.8. Manganese 0.50-1.65 Silicon 0.020.60.Sulfur 0.50 max. Phosphorus 0.040 max. Boron 00005-0007. Iron Balance.

The above example generally characterizes the broad range of compositionfor the present invention. For example, any increase of carbon beyond0.80 percent appears to detract from the advantage of the boron, namely,hardenability. Likewise, as the boron content exceeds the approximateratio set forth above, a resulting decrease in notch toughness tends toappear.

To further characterize the above composition of the present carbonsteel, in view of the definition of carbon steel, it may be furtherspecified that the steel should include no more than 0.40 minimumpercent copper and no more than 0.60 maximum percent copper.

Preferred approximate limits for a carbon-manganeseboron steel accordingto the present invention are as follows:

Elements: Percent Carbon 0.55-0.65. Manganese 1.10-1.40. Silicon0.02-0.30. Sulfur 0.050 max.

Phosphorus 0.040 max. Boron 00005-0003. Iron Balance.

Steels of this composition may be produced, for example, in flat bars,wire, cold finished or hot finished bars fabricated by generallycustomary techniques. For example, hot-coiled springs may be heated to1600 F., coiled, oil-quenched and tempered immediately thereafter. Theymay also be coiled, cooled, reheated to 1,550 F. and then oil-quenchedfollowed by a tempering operation at 700 F. Cold-coiled springs may beheated to approximately 1550 F., quenched and then tempered also at 700F.

Typical properties for recoil spring stock formed with the abovecomposition are as follows:

Hardness: Rockwell-C42-48 Tensile strength: 200,000-250,000 p.s.i. Yieldstrength: 180,000-220,000 p.s.i. Grain size (-8 McQuaid-Ehn): Fine Thesubject composition also possesses improved notched tensile strength,notch toughness and fatigue properties as compared with conventionalspring steel stock.

When either the broad composition range or the preferred composition asset forth above is to be employed for heat-treated parts, the siliconcontent is preferably held to approximately .02-0.30 percent with thesteel being preferably characterized as having a fine grain.

To form wire spring stock up to approximately 1.5 inches in diameter andleaf spring stock up to approximately 1.0 inch in thickness, themanganese composition of about 1.10-1.40 in the preferred compositionmay be maintained. However, for wire spring stock over 1.5 inches indiameter up to approximately 1.875 inches in diameter, as well as forleaf spring stock in the approximate thickness range of 1.0 to 1.25inches, the manganese content of the preferred composition is held tothe approximate range of 1.20-1.40 percent.

What is claimed is:

1. Carbon steel spring stock in wire form of approximately 1.500-1.875inch diameter and in leaf form of approximately 1.00-1.250 inchthickness, consisting essentially of carbon about 0.55-0.65 percent,manganese about l.20-1.40 percent, sulfur about .05 percent maximum,phosphorus about .04 percent maximum, copper about 0.60 percent maximum,silicon about .25-.45 percent, boron about .0005-.003 percent, balanceessentially iron.

2. Carbon steel spring stock of claim 1 which includes from about .4 to.6 percent copper.

3. The carbon steel spring stock of claim 1 wherein the spring stock isoil quenched from a temperature range of from about 1550 F. to 1600 F.and tempered at a temperature on the order of about 700 F.

References Cited UNITED STATES PATENTS 1,992,905 2/ 1935 Wills -1232,280,283 4/1942 Crafts 75-123 2,527,731 10/1950 Ilacqua 267-1 2,542,2202/1951 Urban 75-123 2,861,908 11/1958 Mickelson 148-31 OTHER REFERENCESD. K. Bullens, Steel and Its Heat Treatment, vol. III,

John Wiley and Sons, =Inc., New York, 1949, pp. 64-71.

HYLAND BIZOT, Primary Examiner J. E. LEG RU, Assistant Examiner US. Cl.X.R.

